This invention relates generally to cable and pipe burial apparatus, and more particularly to jetting tools for cable and pipe burial.
The growing demand for international voice and/or data transmission has created the need for more underwater cables to carry this information across the seas. Cables are typically laid in trenches in order to protect the cable from damage and to resist unwanted movement of the cable, which can also result in damage. These trenches have been created primarily by two types of machines, plows and jetting systems. Plows are mechanical devices which are remotely operated and dragged or otherwise propelled across the sea floor. A plow blade extends into the sea floor and creates the trench as the plow is moved. The cable is typically laid simultaneously in the trench that is formed behind the plow. These plows are difficult to operate in deep water, although plows are currently preferred for longer runs. Water jetting systems are preferred in deep water systems.
Trenches for cable and pipes are often excavated in the ground or sea floor using water jetting machines. These machines are equipped with water discharge nozzles mounted on jetting arms. The jetting arms allow the nozzles to shape an approximately vertical sided xe2x80x9cUxe2x80x9d shaped trench in which to bury the cable or pipe. The jetting arms straddle the cable and, as the trenches form, the cable drops in the trench. Jetting is sensitive to the type and strength of soils. These machines typically have a relatively narrow range of soil conditions in which the jetting systems operate efficiently. The nozzles are mounted at fixed points on the jetting arms. The energy requirement of the excavation process in any given soil condition depends on the water discharged through the nozzles, and the spacing between the nozzles on the arms. The jet is created by a motor which typically has constant speed. The discharge nozzle diameter is difficult to adjust under these conditions, as is the spacing between the nozzles on the arms. Consequently, current jetting machines are severely limited in their ability to operate in variable soil conditions.
Variable soil conditions are encountered frequently in the sea floor. Packed soils containing aggregates are more difficult to remove by water jets than are looser, fine soils. These differing soil conditions from trenches require different jetting characteristics in order to adjust efficiently and effectively. It has proven difficult, however, to make these adjustments. The ability of water jetting equipment to make a trench in any given soil type which may be encountered depends on the power available, water discharge through the nozzles, and the spacing between the nozzles on the arms. Consequently, when the energy available, water discharge and nozzle spacings are all fixed the machine is limited in its ability to operate in varying soil conditions. It is currently uneconomical to manufacture machines with variable energy and water discharge capability. The productivity of these machines tends to reduce dramatically when soil conditions vary from those assumed in design, which is frequently the case.
Many cables and pipes are laid in tight loops on the sea floor prior to burial. This requires the jetting machine to turn frequently. Current jetting systems typically rotate the entire jetting arm around a single hinged point as a means for altering trench depth. This restricts the ability of the jetting machine to turn efficiently.
It is preferable to alter the trench depth as soil conditions vary. Stronger soils afford greater protection, and a shallower trench is necessary than where soils are weaker, and thus where burial must be deeper. A uniform level of protection against an aggressive strike with varying trench depth in varying soil strength is known as the Burial Protection Index (BPI) and is discussed by Mole, et al., xe2x80x9cCable Protection-Solutions Through New Installation and Burial Approaches,xe2x80x9d Suboptice 1997, San Francisco, May, 11-16 1997. The BPI is a measure of the level of protection which must be afforded against different physical threats to the cable or pipe, such as trawling or anchors. More vigorous threats, such as anchors, require a higher BPI. Different soils provide the same BPI number at different trench depths. Stronger soils generally provide the same BPI protection with a shallower trench depth than do weaker soils.
Obstructions are sometimes encountered by burial systems. Accordingly, it is known to make nozzles retractable in case of obstructions. Such a system is shown in FIGS. 2A and 2B. The nozzles 10, 18 and 24 are provided on the jetting assembly 14. The nozzle 24 is typically extended by assembly 28, while the nozzles 10 and 18 are fixed in position. Only the position of the nozzle 24 is adjustable. Retraction of the assembly 28 is possible from the position shown in FIG. 2B to the position shown in FIG. 2A in the event that an obstruction is encountered. The vertical position of the nozzles 10 and 18 is not adjustable, and accordingly, this system is not useful to vary the depth of the trench that is formed.
An apparatus for burying cables and pipes in soil includes a support structure adapted for movement across a surface of the soil. A source of pressurized water is provided. At least two jetting arms in fluid communication with the source of pressurized water have vertical position adjustment structure for varying at least the vertical position of each jetting arm relative to the support structure. The vertical position adjustment structure is preferably independently adjustable.
At least two vertically adjustable jetting arms are preferably aligned as a row substantially in the direction of movement of the support structure. First and second rows of the vertically adjustable jetting arms are most preferably provided and are laterally spaced at a distance to result in the desired trench width.
A method for burying cables and pipes in soils comprises the steps of moving at least two water jets across the sea floor and independently varying the vertical height of the water jets depending on soil conditions. In a preferred embodiment, at least two rows of jets are provided in spaced relation to define a trench width. The cable is dropped into the trench behind the jets.